The present invention pertains to continuous cast Cuxe2x80x94Nixe2x80x94Sn spinodal alloys, and more particularly to a method for producing a continuous cast Cuxe2x80x94Nixe2x80x94Sn spinodal alloy wherein it is unnecessary to subject the billet or rod to wrought processing prior to the spinodal heat treatment. The Cuxe2x80x94Nixe2x80x94Sn spinodal alloy is characterized by a substantial absence of discontinuous xcex3xe2x80x2 phase precipitate at its grain boundaries. The symbol xcex3xe2x80x2 corresponds to the metastable coherent discontinuous precipitate referred to in the prior art having elevated percentages of Ni and Sn. The superscript distinguishes xcex3xe2x80x2 from xcex3 which is also an elevated percentage Ni and Sn precipitate which is both stable and incoherent and differs from xcex3xe2x80x2 in that it does not cause embrittlement while adding strength.
It has been known to be beneficial to all casting and metal working schemes to have the grain boundaries be as thin and grain size as small as possible. For this reason, it was considered desirable to develop an arrangement which would readily facilitate obtaining such fine grain structures in continuous cast copper alloy rods and tubes. Such rods and tubes would then satisfactorily accommodate subsequent cold drawing or working or would exhibit better properties than other unwrought materials. Thus, U.S. Pat. No. 4,315,538 disclosed a method and apparatus to effect a fine grain size in continuous cast metals. This method involved the use of a continuous casting die totally submerged in a reservoir of liquid alloy material and the use of feed openings in the die arranged so that the liquid metal entering the die would impart a generally cyclonic motion at the interface zone between the liquid and solid alloy material. This cyclonic motion caused shearing of primary dendrites in the alloy material from adjacent the internal side wall of the die and distributed those dendrites across the interface zone to provide nuclei for equiaxed crystals, thereby preventing the formation of thermal gradients in the alloy material of a sufficient magnitude to produce gross directional solidification at the interface zone.
The subject of U.S. Pat. No. 5,279,353 was a die construction for use with the same type of continuous casting apparatus, but with an improved ability to produce a fine grain structure in tubes with wall thicknesses greater than 0.5 inch, as well as in other cast shapes, such as round shaped rods, billets, or non-round rods and billets. We estimate the grain size of the resultant cast shape to be greater than 20 xcexcm, possibly as large as 40 xcexcm, but still substantially smaller than shapes cast by other means.
We have subsequently discovered that an additional benefit for producing copper metallic alloys according to the continuous cast method set out in our U.S. Pat. No. 5,279,353, said metallic alloys composed of small, equiaxed crystals, relates to the production of copper alloys requiring spinodal decomposition type phase transformation to achieve desired physical properties.
Spinodal decomposition type phase transformation in a multicomponent alloy system is described in U.S. Pat. No. 3,806,336 issued Apr. 23, 1974; U.S. Pat. No. 3,954,519 issued May 4, 1976; and U.S. Pat. No. 4,171,978 issued Oct. 23, 1979. As described in those patents, a certain binary and other metallic has, in its composition diagram, a xe2x80x9climit of metastabilityxe2x80x9d or xe2x80x9cspinodalxe2x80x9d which is thermodynamically defined as the locus of disappearance of the second derivative of the chemical free energy with respect to composition of the system. When a high-temperature composition, which is of homogeneous single-phase structure, of the alloy is brought within the spinodal in a low temperature range, it is transformed into a separated two-phase structure, the phase separation being called spinodal decomposition. The decomposed alloy has a periodic microstructure generally in the order of hundred of angstroms and which consists of composition modulated two isomorphous phases in which one phase is in the form of a fine precipitate uniformly distributed in another phase which forms the matrix.
It is known that an alloy requiring a spinodal transformation must have a homogeneous composition throughout the entire alloy. Within the homogeneous volume, it is possible, by thermal treatment, to cause a shift in atomic concentration of certain of the solute metals comprising the alloy. Such a change, spinodal decomposition, imparts new physical properties to the alloy.
One skilled in the art is aware that microsegregation of solute elements results in areas exhibiting various responses to spinodal heat treatments. Typically, continuous cast billets or other castings always exhibit gross inverse segregation as well as xe2x80x9ccoringxe2x80x9d or microsegregation within dendritic cells. To render such billets or castings fit for spinodal treatment, the metal parts have in the past been subjected to wrought processing to reduce microsegregation by xe2x80x9ckneadingxe2x80x9d the material to mechanically reduce the secondary inter-dendritic distances. The wrought processing typically involves rolling, drawing, or pilgering to reduce the cross-sectional area dimensions by 40-90%. When very large degrees of cold working are employed to effect a 40-90% reduction in the cross-sectional area, however, it is very costly or even impossible to produce alloy parts large enough to function in many applications.
Further wrought processing cannot overcome the gross inverse segregation exhibited by alloys that have wide freezing ranges. Because these alloys have wide freezing ranges, concentration fluctuations of solute elements over a given distance within the body of the alloy are too large to effectively eliminate by solution heat treatment; consequently, these alloys will not spinodally decompose; furthermore, they are susceptible to forming other embrittling metastable phases via discontinuous precipitation.
By use of our continuous cast process to form billets, rods, or tubes, in combination with our method for manufacturing the Cuxe2x80x94Nixe2x80x94Sn spinodal alloy, it is now possible to avoid all of the aforementioned difficulties of the processes disclosed in the past, particularly the need to subject the metal parts to wrought processing prior to spinodal decomposition. A rod or tube manufactured according to our continuous cast process advantageously has the following characteristics: (1) uniform solute distribution from surface to center and (2) a greatly reduced secondary inter-dendritic arm spacing, which cannot be achieved by conventional means. The secondary inter-dendritic arm spacing of the crystals manufactured in accordance with our process is only one-tenth or less than that for other materials. Consequently, fluctuations in the solute element concentration in the homogenized alloy are minimized, thereby permitting the spinodal transformation of a greater volume of the alloy while simultaneously avoiding precipitation of other undesirable metastable phases which have an adverse effect on ductility and toughness. Further, by use of our method disclosed for manufacturing the Cuxe2x80x94Nixe2x80x94Sn spinodal alloy, the resultant spinodal alloy is characterized by a substantial absence of discontinuous xcex3xe2x80x2 phase precipitate at its grain boundaries. In this regard, it is now possible to produce rods having a cross-section greater than xe2x85x9c inches, which can subsequently be thermally processed in an unwrought condition to develop high strength and ductility and are therefore suitable in the production of various components which include, among other things, journal bearings, wear plates, mold plates, and gravure printing rolls.
U.S. Pat. No. 5,279,353 is incorporated by reference herein as background information with respect to the present invention.
The present invention provides a Cuxe2x80x94Nixe2x80x94Sn spinodal alloy which is continuous cast in such a manner as to effect small, equiaxed crystals, and subjected to various heat and aging treatments to effect spinodal decomposition type phase transformation without the need for wrought processing.
According to a first aspect of the invention, an unwrought continuous cast Cuxe2x80x94Nixe2x80x94Sn spinodal alloy is disclosed which comprises from about 8-16 wt. % nickel, 5-8 wt. % tin, and a remainder copper. The alloy is further characterized by a substantial absence of discontinuous xcex3xe2x80x2 phase precipitate at its grain boundaries and ductile fracture behavior during tensile testing.
According to a second aspect of the invention, a method for manufacturing an unwrought continuous cast Cuxe2x80x94Nixe2x80x94Sn spinodal alloy is disclosed. The Cuxe2x80x94Nixe2x80x94Sn alloy is continuous cast in such a manner as to effect small, equiaxed crystals. The alloy is subjected to a solution heat treatment at a predetermined optimal temperature for a predetermined length of time to transform the matrix of the alloy to a single phase and immediately quenched with cold water. The resultant alloy is then subjected to a spinodal decomposition (aging) heat treatment at a predetermined optimal temperature and for a predetermined length of time and again immediately quenched with cold water.
According to a third aspect of the invention, a method for determining optimal temperatures and times for a heat treatment used in the manufacture of an unwrought continuous cast Cuxe2x80x94Nixe2x80x94Sn spinodal alloy is disclosed. An unwrought continuous cast Cuxe2x80x94Nixe2x80x94Sn alloy having small, equiaxed crystals is provided. A first sample of the alloy is subjected to a first solution heat treatment immediately followed by quenching, preferably with cold water. The first alloy sample is next subjected to a first spinodal decomposition heat treatment immediately followed by a step of quenching with an aqueous medium such as cold water. A second sample of the alloy is subjected to a second solution heat treatment immediately followed by a step of quenching, preferably with cold water. Thereafter, the second alloy sample is subjected to a second spinodal decomposition heat treatment immediately followed by a step of quenching with an aqueous medium such as cold water. The two (2) spinodal alloy samples are metallographically examined to determine the optimal temperatures and corresponding times for an optimal heat treatment which comprises both a solution heat treatment and a spinodal decomposition heat treatment. The optimal heat treatment yields a Cuxe2x80x94Nixe2x80x94Sn spinodal alloy having optimal hardness and ductility.
One advantage of the present invention is that a Cuxe2x80x94Nixe2x80x94Sn spinodal alloy having both strength and ductility can be produced without the need for wrought processing to reduce microsegregation within dendritic cells.
Another advantage of the present invention is that a continuous cast billet or rod is manufactured having uniform solute distribution from surface to center which can be subjected to spinodal decomposition without the need for wrought processing.
Still another advantage of the present invention is that a continuous cast billet or rod is manufactured having secondary inter-dendritic arm spacing of one-tenth or less than that of typical continuous cast materials which can be subjected to spinodal decomposition without the need for wrought processing.
Still another advantage of the present invention is that an unwrought Cuxe2x80x94Nixe2x80x94Sn spinodal alloy is simply and economically manufactured.
Still another advantage of the present invention is that an unwrought continuous cast Cuxe2x80x94Nixe2x80x94Sn spinodal alloy having optimal mechanical properties can be produced.
Still another advantage of the present invention is that an unwrought Cuxe2x80x94Nixe2x80x94Sn spinodal alloy being essentially free of a discontinuous xcex3xe2x80x2 phase at the grain boundaries can be produced.
Still another advantage of the present invention is that heavy objects having a cross-section greater than xe2x85x9c inches for a rod and up to 16 inches for a billet can be thermally processed in an unwrought condition to yield high strength and ductility.
Still another advantage of the present invention is that an unwrought Cuxe2x80x94Nixe2x80x94Sn spinodal alloy can be produced which is able to reach and exceed necking strain prior to fracture during tensile testing.
Still another advantage of the present invention is that an unwrought Cuxe2x80x94Nixe2x80x94Sn spinodal alloy can be produced which exhibits ductile fracture behavior during tensile testing.
Still another advantage of the present invention is that the higher temperatures used for heat treating the unwrought Cuxe2x80x94Nixe2x80x94Sn allow reduction in the time the billet must remain in the furnace thereby resulting in improved financial savings as well as improved productivity.
Still a further advantage of the present invention is that a wide array of products can be simply and economically manufactured using the Cuxe2x80x94Nixe2x80x94Sn spinodal alloy prepared by the disclosed processing. Such products include, among other things, bearings, gears, and other wear parts such as aircraft landing gear bearings, stamping press wear plates, molds and other components for die casting or plastic injection, heavy equipment bearings, and fluid power transmission system components.
Still other benefits and advantages of the invention will become apparent to those skilled in the art upon a reading and understanding of the following detailed specification.